The present invention relates to electromagnetically deflected cathode-ray-tubes and particularly to apparatus for correcting distortion of an electron beam as caused by the magnetic field of an electromagnetic deflection yoke.
Electromagnetically deflected cathode-ray-tubes suffer from distortion of the spot written by the tube's electron beam on the CRT screen as a result of nonuniformities in the magnetic deflection field. Thus, when a magnetic deflection yoke in energized to deflect the tube's electron beam from its center position to a more peripheral position, the electrons in the electron beam are deflected somewhat differently depending upon their position in the beam. As a result the electron beam becomes defocused despite the presence of focusing means in the electron gun that generates the beam. As the beam is deflected radially outwardly from the center screen position, the spot tends to become elongated in a direction perpendicular to the direction of deflection. Thus, if the beam is deflected horizontally, to the right of center screen, the spot will tend to be elongated in a vertical direction. Although the astigmatism distortion is not sufficiently serious to prevent the use of electromagnetically deflected cathode-ray-tubes for many purposes, this distortion can be of considerable concern where finely detailed information is to be presented, e.g. on the screen of a computer terminal portraying a great deal of graphic and alphanumeric information. A raster scan may be employed in the case of CRTs used in these applications wherein the raster is composed of a large number of lines as compared with the conventional TV application.
A prior art apparatus for correcting the astigmatism caused by an electromagnetic deflection yoke is described in Ray et al U.S. Pat. No. 3,961,223. This system employs a set of correction coils wound upon a separate annular magnetic core positioned around the neck of the cathode-ray-tube. Current is supplied to this set of coils, via amplifiers for converting correction voltages to coil currents. In a first pair of coils, a current is provided which is proportional to (Vx).sup.2 -(Vy).sup.2, where Vx and Vy are horizontal and vertical deflection signals, while a second set of coils is provided a current proportional to 2VxVy. Electromagnetic correction devices and the circuitry therefor tend to be expensive and cumbersome not only in respect to the cost of the equipment but also in regard to proper adjustment external to the cathode-ray-tube. Moreover, the correction achieved is somewhat approximate and does not take into account the considerable differences in the astigmatic distortion produced by different magnetic deflection yokes.